1. Field of the Invention
The invention relates to an improved method and apparatus for repairing or reworking complex multilayer printed circuit board interconnections; and more particularly to a system of repairing circuit net connections using ball grid array contacts.
2. Description of the Prior Art
It is known to utilize printed circuit boards to make electrical interconnections between leads of electrical components mounted thereon, including mounted integrated circuits. Printed circuit boards characteristically involve one or more discrete layers of insulating material upon which patterns of electrical conductors are formed in conjunction with a predetermined array of holes. The electrical conductors are referred to as foil. The layers are stacked, bonded, and the hole patterns formed and plated-through with electrical conductive materials. Characteristically, plated-through holes are uniform in diameter, and are often referred to as xe2x80x9cbarrelsxe2x80x9d. Selective interconnections result in selected wiring networks, or nets, being formed with various ones of the electrical conductors on different layers being interconnected through plated-through holes. This effectively provides a three-dimensional wiring system. At the mounting surfaces, it is known to provide additional electrically conductive material in electrical contact with the plated-through holes for the purposes of providing an expanded pad area for making interconnection with the component contacts, terminals, leads, or mounting contacts.
It has been the usual practice to have the diameter of the plated-through holes uniform throughout the thickness of the multilayer printed circuit board. With the advent of continued reduction in size of the electronic components, the spacing of the component connections has been decreasing, and the density of the associated array of holes has been ever-increasing. As the density of integrated circuit components increase the array of interconnection points to a printed circuit board multiplies for the same integrated circuit die size. At the same time, the complexity of the interconnections that are to be made by the multilayer printed circuit boards has increased. This increase in the number of interconnections results in the requirement of providing more conductive routing paths on the various layers, where these routing paths must be constructed in ever-decreasing dimensions. The loss of routing area resulting from the reduction in pin spacing in the grid array of plated-through holes has lead to the requirement of adding additional layers to the multilayer printed circuit board assemblies, with the attendant increase in cost of manufacture.
It is known to utilize so-called surface mount components, where relatively short component leads are affixed to surface pads on the multilayer printed circuit boards. Characteristically, they are affixed through a solder process. The surface mount has the problem of location of all of the terminals of a component over the associated interconnection pads during the soldering process. Any misalignment or misplacement can result in missing or marginal solder interconnections.
A form of surface mount includes the process of mounting packaged integrated circuits (IC) to printed circuit board assemblies having an array of pads on the printed circuit board interconnected to a matching array of IC contacts through a soldering process that uses solder balls. This interconnected technology is generally referred to as a ball grid array (BGA).
It is also well-known in the design of the electrical interconnections to be made on the printed circuit board assembly, that design problems can result in wiring network layouts that must be altered or reworked to form correct wiring network interconnections. Further, it is known that in the fabrication of multilayer printed circuit boards it can occur, for various manufacturing reasons, that one or more layers may have electrical conductors improperly electrically shorted to a plated-through hole. Unless such re-routing or shorted condition can be repaired, the entire assembly has to be scrapped. Various types of rework and repair techniques have been developed, but such known repair and rework techniques are complex; and if not accomplished with skill and precision, can result in further damage to the printed circuit board and failure to remedy the problem. As the array of printed circuit board interconnections become larger there is an increase in the number of plated-through holes of smaller diameter, and more layers, are utilized, it becomes more and more necessary to have an effective and efficient way to rework incorrect or defect wiring nets to save the boards.
The development of integrated circuits (ICs), including application specific integrated circuits (ASICs), requires on-circuit interconnection of cells and requires off-circuit interconnections to connect circuits on different ICs or to connect to connectors or other components, power, ground, and the like. When design errors, manufacturing errors, or design changes occur relative to interconnection of IC pins, the entire supporting printed circuit board must be scrapped unless a repair system is provided.
For those systems where ICs or ASICs were interconnected through metal pins soldered into plated-through holes in the printed circuit boards or to pads on the surface of the printed circuit boards it was possible to solder the repair wire directly to the pin or surface pad. Those resoldering processes usually required the pin to be disconnected from the wiring net within the printed circuit board to be accomplished by drilling out the via hole in the printed circuit board to disconnect the internal wires. It has been determined, however, that structures that utilize BGA surface contact solder connection to interconnect the ICs or the ASICs to the printed circuit board that such a drilling out and resoldering process cannot be accomplished within the tolerances of the structure. Further, the drilling out process requires very close tolerance and often results in destruction of the solder pad.
Accordingly, it is desirable to have a repair system and method that overcomes the limitations for repair incident in the prior art.
It is the primary objective of this invention to provide a method and apparatus to efficiently repair and rework multilayer printed circuit boards.
It is a further primary objective of this invention to provide an improved printed circuit board assembly having an improved structure for use in affixing electrical components that may be reworked as necessary to correct design and manufacturing deficiencies.
Yet another objective of the invention is to provide an improved printed circuit board assembly having a structure for affixing electrical components that is suitable for use with BGA interconnections.
Another objective of the invention is to provide an improved multilayer printed circuit board assembly having an array of plated-through holes to interconnect selected layers, and a rework structure that can be utilized to rework circuit interconnections with selected ones of the plated-through holes.
A further objective of the invention is to provide an improved multilayer printed circuit board assembly for use with an enhanced capability of rework or repair to allow repair of defects in manufacture and rework of erroneous interconnections of electrical wiring networks with improved rework structures.
Still a further objective of the invention is to provide an improved repair and rework pad structure for use with a multilayer printed circuit board assembly for effectively and reliably allowing the repair and rework of the assembly to accommodate solder interconnections with integrated circuits.
Yet another objective of the invention is to provide an improved reworked pad structure that matches the shape of a removed pad, has an insulating material deposited on a portion thereof, and a conductive portion that interacts with a BGA solder pad.
A further objective of the invention is to provide an improved rework apparatus and method to efficiently and economically rework multilayered printed circuit boards to avoid the necessity to scrap defective or damaged boards.
Other more detailed objectives will become apparent from a consideration of the Drawings and the Detailed Description of the Preferred Embodiment.
To overcome the problems in the prior art and improved repair and rework method have been developed wherein the repair and rework method is applied to rework a circuit connection on a multilayer printed circuit board that mounts one or more integrated circuits and have a predetermined array of BGA solder pads to interconnect with each integrated circuit, wherein the array of pads are electrically connected to respectively associated via lands on plated-through hole barrels on a first surface of a printed circuit board, where each of the barrels extends through the printed circuit board to a second surface. The barrels are formed as plated-through holes and interconnect to predetermined wiring nets within the printed circuit board. The repair method involves disassembling the integrated circuit from the ball grid array of solder pads to which the integrated circuit is electrically connected, to thereby expose the array of solder pads. A particular pad is identified to be repaired, and the identified pad is removed along with its associated via pad and interconnecting foil. A repair wire is selected and placed in the associated barrel of the plated-through hole from which the identified solder pad has been removed, with the insertion being such that a first end is extended past the first surface of the printed circuit board and is available for forming. Once installed, the exposed end of the repair wire is formed into a shape to approximate that of the removed solder pad. The formed pad is positioned and bonded to the first surface of the printed circuit board. The second end of the repair wire is routed and connected to the appropriate corrected circuit connection to complete the rework and repair. The entire assembly is then reassembled with the removed integrated circuit brought into electrical interconnection with the array of solder pads, including the newly formed pad formed on the repair wire.
For certain repair functions it, is desirable to provide a selective coating on the newly formed solder pad on the end of the repair wire. By way of example, it may be necessary to coat the replacement repair contact pad with gold to match the performance of the other pads in the ball grid array and to treat the repair pad with solder so that it will properly make soldered connection when subjected to the solder ball soldering process.
An improved repaired assembly has been provided and includes a multilayer printed circuit board having first and second surfaces; an array of BGA solder pads arranged on said first surface to electrically interconnect associated contacts on at least one integrated circuit mounted on the multilayer printed circuit board; a plurality of electrical wiring nets in the multilayer printed circuit board; a plurality of plated-through holes extending through the printed circuit board between the first and second surfaces, with selected ones of the plurality of plated-through through holes in a electrical contact with associated ones of the plurality of wiring nets, and each of the plurality of plated-holes electrically connected to an associated different one of the array of the BGA solder pads; a repair wire having a first end portion formed in the shape of a solder pad and positioned on the first surface of the printed circuit board to replace a removed one of the array of solder pads, and the repair wire having a board transition portion extending through the one of the plated-through through holes that is associated with the removed one of the pads, and the repair wire having a second end portion extending beyond the second surface and making circuit connection to a selected circuit interconnection point to repair the function of the circuit.